Gas discharge lasers such as excimer lasers are well known light sources useful for integrated circuit lithography. These lasers typically comprise two elongated electrodes (for example, about 50 cm in length) separated by about xc2xe inch. A high voltage pulse power source provides high voltage electrical pulses to produce discharges between the electrodes to create a gain region in a circulating laser gas. A tangential fan is provided to produce sufficient laser gas flow to remove from the discharge region substantially all debris produced by each discharge prior to the next succeeding discharge. The following four patents assigned to Applicants"" employer describe prior art tangential fan designs: U.S. Pat. Nos. 6,034,984, 6,061,376, 6,195,378 B1 and 6,144,686. These patents are incorporated herein by reference.
FIGS. 1a and 1b are cross-sectional end and side views respectively showing the inner structure of a laser chamber 100 in a conventional excimer laser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990, and incorporated herein by reference in its entirety). A laser enclosure 102 provides isolation between a laser chamber interior 105 and the exterior 110. Typically enclosure 102 is formed by a pair of half enclosure members 112 and 114 (see FIG. 1a), which are coupled together and sealed using an o-ring seal 116, extending along a perimeter of enclosure 102. Laser chamber interior 105 is filled to a predetermined pressure with a lasing gas 108. A pulsed gas discharge is generated in a discharge region 122 by a high voltage pulse applied between a cathode assembly 118 and an anode assembly 120. The pulsed gas discharge typically produces excited argon fluoride, krypton fluoride, or fluorine molecules, which generate laser pulse output energy. The pulse output energy propagates from discharge region 122 through an optical output window assembly 162 (see FIG. 1b). Cathode assembly 118 and anode assembly 120, defining discharge region 122, extend parallel to one another along the length of laser chamber 100.
Recirculation of lasing gas 108 is provided by a tangential fan 140, which rotates about an axis 142 and includes a plurality of substantially parallel straight blade members 144 extending along the length of laser chamber 100 between hub members 146. A typical rotation rate for current tangential fans is on the order of approximately 3800 revolutions per minute (rpm). As shown by arrows in FIG. 1a, the flow of gas 108 is upward through tangential fan 140 and transversely across discharge region 122 as directed by a vane member 152. Lasing gas 108 that has flowed through discharge region 122 becomes dissociated and heated considerably by the pulsed gas discharge. A gas-to-liquid heat exchanger 158 (not shown in FIG. 1b) extending along the length of laser chamber 100 is positioned in the gas recirculation path to cool the heated gas. Other vane members, e.g. vane members 160, direct the flow of gas 108 through heat exchanger 158 and elsewhere along the gas recirculation path. Recirculation cools and recombines lasing gas 108, thereby allowing repetitively pulsed laser operation at rates in the range of 2,000 to 4,000 Hz.
Since lasing gas 108 is recirculated and reused, it is important to maintain cleanliness and to prevent contamination of the gas environment within laser chamber interior 105, in order to maximize the pulse energy performance, stability, and working life of lasing gas 108.
In excimer lasers of the type referred to in there patents, the electric discharge produces acoustic waves which can reflect back to the discharge region at a time coincident with a succeeding pulse, especially the next succeeding pulse. These returning shock waves can disturb the gain region produced by the succeeding pulse and adversely affect the quantity of the laser pulse produced in the gain region. One of the important improvements provided by the fans described in the above patents was to vary the circumferential position of the blades to reduce cylindrical symmetry of the fan blades.
Thus, in one of the designs a helical blade pattern was provided. In another design, the fan was segmented and the blades in each segment were offset from each other. Designs were also proposed to improve the stiffness of the fan blade structure. These and other improvements were described in these patents to avoid aerodynamic buffeting effects which could reduce bearing life and adversely affect beam quality.
The improvements described in the above patents provided substantial improvements in laser performance including beam quality. However, Applicants have discovered that even after incorporating the above described improvements, laser beam quality can be adversely affected when the laser is operating at several combinations of specific discharge repetition rates and specific fan rotation rates.
What is needed is a better tangential fan.
The present invention provides an electric discharge laser apparatus having a laser chamber containing a laser gas and two longitudinal electrodes defining a discharge region for producing electrical discharges and a tangential fan for circulating the laser gas and having blade members configured to minimize adverse effects of reflections of electric discharge generated shock waves back to the discharge region simultaneously with a subsequent discharge.
In preferred embodiments, the blades are arranged in segments in a pattern simulating a double helix. In other preferred embodiments, the blades are arranged in segments with the blades in each segment positioned in an asymmetrical pattern. In other preferred embodiments, the blades in each segment are arranged to simulate a double helix and also arranged asymmetrically with respect to blades in other segments.
In another embodiment, the blades are arranged to simulate the double helix pattern and in addition are arranged in an asymmetrical pattern with respect to the blades in the same segment and/or with respect to blades in other segments.